Power semiconductor components, for example power MOSFETs, power IGBTs, power diodes or power thyristors, are widely used to drive electronic loads. In this case, power MOSFETs, in particular, are used to switch electrical loads.
Such power MOSFETs which are suited to switching electrical loads are, for example, the power MOSFETs in the PROFET family from Infineon Technologies AG, Munich. In addition to the actual power component, these components also contain protective circuits for the power component in the same housing as the power component. These protective circuits may be monolithically integrated in the same semiconductor chip as the power component or may be integrated in a separate chip, which is applied to the power component's chip (chip-on-chip technology), and are used, for example, to protect the component from overtemperature, overvoltage or an excessively high load current. The design and operation of such “intelligent semiconductor switching elements” (smart power switches) are described, for example, in Graf, Alfons: “Smart Power Switches for Automobile and Industrial Applications”, VDE, ETG Conference “Contact Performance and Switching”, Karlsruhe, Sep. 26-28, 2001, or in the data sheets PROFET BTS307, 2003 Oct. 1, and BTS5210L, 2003 Oct. 1, from Infineon Technologies AG, Munich.
If such intelligent semiconductor switching elements are operated at load currents which are above the nominal current and which may arise, for example, if the connected load is shorted, the power loss which is converted into heat in the component rises. This results in an increase in temperature in the component, said increase resulting in a temperature protective circuit responding and the component being switched off.
After a cooling phase, the semiconductor switching element can either be switched on again (“retry” operation) from the outside, for example using a microcontroller, or the intelligent semiconductor switching element is designed in such a manner that it switches itself on again (“restart” operation) after cooling when a prescribed temperature has been undershot.
Semiconductor bodies or semiconductor chips in which such intelligent power components are integrated are usually surrounded by a housing which comprises a molding compound and from which connection legs for contact-connecting the semiconductor component project. Connections, for example bonding wires, are present within the housing between contact zones of the semiconductor body and the connection legs. Permanently switching the power component in a cyclic manner, for example in the event of the load being shorted for a relatively long time, results in thermomechanical stress in the bonding wires and particularly in the transition region between the contact zone and the bonding wire. This thermomechanical stress can result in material fatigue and cracks in the contact zone which is, for example, a metallization which has been applied to the semiconductor body. As a result, the contact resistance between the bonding wire and the contact zone or the contact zone's resistance increases, with the result that the semiconductor body may be severely overheated in this transition region. In the case of power MOSFETs, severe overheating of the semiconductor body may result in the semiconductor body breaking down between the source and drain, with the result that the power component is permanently on even if the gate electrode is not being driven. A comparatively high forward resistance of the component in this damaged state results in a high power loss which is converted into heat and, in extreme cases, may damage surrounding components or a printed circuit board to which the component has been applied. This may have serious consequences for the loads which are connected to the power component.
In order to solve this problem, it is known practice to limit the maximum permissible number of switching-on and switching-off cycles and to completely prevent the component from being driven after this maximum number of cycles has been reached.
Furthermore, the robustness of the components can be enhanced by using as many bonding wires as possible to connect the contact zone to the contact element which is accessible, on the housing, from the outside.
In addition, the temperature value at which the component is switched off on account of overheating can be reduced in order to reduce the thermal stress. Since, however, the nominal current generally determines the maximum permissible power loss of the component during short-circuit operation, the power density can only be reduced by increasing the active component area of the power component, which, however, is not economically viable.
In addition, the temperature sensor which detects the temperature in the component and causes the component to be switched off in the event of an overtemperature could be optimized to the effect that it is positioned in such a manner that the thermal contact resistance between the “hottest” regions in the semiconductor component and the temperature sensor is as small as possible so that the temperature sensor detects the component temperature immediately, as far as possible, in order to keep a delay time between the occurrence of the overtemperature and the response of the sensor as short as possible.